DOAS Ventilation and Load Sizing Calculator
This calculator sizes your dedicated outdoor air system in 3 steps. Enter your space type and occupancy to get ASHRAE 62.1 ventilation CFM. Add your project location to get cooling tonnage through the enthalpy method and heating MBH through sensible heat.
Standard BTU calculators treat outdoor air like recirculated air. DOAS units handle 100% outside air across the full range of ambient conditions. That means latent loads in summer and extreme temperature differentials in winter.
Generic tools miss both.
You get ventilation, cooling, and heating loads from one tool, built around the formulas your specs already require.
Your DOAS System Requirements
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Supply Air Strategy
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How Much Outside Air Does Your Space Need?
Your DOAS unit supplies 100% outdoor air. The required volume depends on how many people occupy the space and how large it is. ASHRAE Standard 62.1 defines minimum ventilation rates for both variables.
ASHRAE 62.1 Ventilation Rate Formula
Component Area
Component Ventilation
Effectiveness
| Variable | Definition | Unit |
|---|---|---|
| Rp | Per-person ventilation rate | CFM/person |
| Pz | Number of occupants in the zone | People |
| Ra | Per-area ventilation rate | CFM/sq ft |
| Az | Zone floor area | Square feet |
| Ez | Ventilation effectiveness (adjusts for supply air distribution) | Decimal |
Exhaust-match sizing pulls a number from hood specs and stops there. The 62.1 method sizes ventilation to actual occupant density and space function. A 5,000 square foot restaurant dining room and a 5,000 square foot warehouse need vastly different outdoor air volumes. This formula captures that difference.
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How Do You Calculate Cooling Load for 100% Outside Air?
A standard AC system recirculates indoor air and only cools a small temperature gap. A DOAS unit conditions raw outdoor air from ambient down to supply temperature. That means your cooling load must account for moisture, not just heat.
DOAS Cooling and Dehumidification Formula
This formula calculates total cooling load using enthalpy, which captures both heat and moisture in a single value.
Heat Rate
Airflow
Enthalpy
Difference
| Variable | Definition | Unit |
|---|---|---|
| 4.5 | Air density × 60 minutes (converts CFM to pounds per hour) | Constant |
| CFM | Ventilation airflow from Panel 1 or your own value | Cubic feet per minute |
| h_outside | Enthalpy of outdoor air at summer design conditions | BTU/lb |
| h_supply | Enthalpy of supply air (default 23.2 BTU/lb at 55°F saturated) | BTU/lb |
| 12,000 | BTU per ton of refrigeration | Conversion factor |
The enthalpy method captures both sensible heat and latent moisture in a single calculation. Temperature-only formulas undersize DOAS equipment in humid climates because they ignore the energy required to condense water vapor. A 93°F day in Chicago carries far more cooling load than a 93°F day in Denver. Enthalpy accounts for that gap. Your heated and cooled make-up air unit specs depend on getting this number right.
How Do You Calculate Heating Load for a DOAS Unit?
Heating load for a DOAS unit is a sensible-only calculation. No moisture variable needed. You measure the temperature difference between your target discharge and the coldest outdoor air your system will handle.
DOAS Heating Load Formula
Difference
This formula calculates the heat energy required to raise 100% outdoor air from winter design temperature to your discharge setpoint.
| Variable | Definition | Unit |
|---|---|---|
| 1.08 | Air density × specific heat × 60 minutes (converts CFM to BTU/hr per °F) | Constant |
| CFM | Ventilation airflow from Panel 1 or your own value | Cubic feet per minute |
| T_discharge | Target supply air temperature leaving the unit | °F |
| T_outside | Winter design temperature for your location | °F |
| 1,000 | BTU per MBH (thousands of BTU per hour) | Conversion factor |
Winter design temperatures vary dramatically by region. Chicago uses -6°F while Houston uses 28°F. That 34-degree spread changes your heating capacity requirement by more than 50% at the same CFM. The calculator pulls ASHRAE-based winter design data for ten major U.S. cities so your heating load matches real winter conditions.
When You Need More Than a Calculator
This tool gives you preliminary sizing numbers. It does not replace a full engineering review. Mixed-use buildings, variable occupancy zones, and projects with energy recovery all require analysis beyond what a calculator can deliver.
We spec DOAS units across multiple manufacturer lines and match equipment to your application, not a catalog. Every recommendation includes engineering review, selection assistance, and fast quoting.
If your project needs more than a number, we can help.
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